The invention relates to a reflective film comprising a layer of a polymerized mesogenic material with a helically twisted orientation, wherein the helix axis is perpendicular to the film, and containing regions with varying helical pitch. The invention further relates to a process of preparing such a reflective film that allows to control the pitch varation. The invention further relates to the use of such a reflective film in optical, electrooptical, information storage, decorative and security applications, and to a liquid crystal display comprising such a reflective film.
Reflective films comprising cholesteric liquid crystal materials have been proposed in prior art for a variety of uses, inter alia for use as broadband or notch polarizers, as colour filters in displays or projection systems, and for decorative purposes like e.g. for the preparation of coloured image films or cholesteric pigment flakes.
These films usually comprise one or more layers of a cholesteric liquid crystalline material with a helically twisted orientation, wherein the helix axis is perpendicular to the film plane, and show selective reflection of light
The bandwidth xcex1,xcex of the waveband reflected by a reflective film as described above is depending on the birefringence of the mesogenic material xcex94n and the pitch of the molecular helix p according to the equation xcex94xcex=xcex94nxc3x97p. Thus, the bandwidth among other factors is determined by the birefringence of the material.
For an application e.g. as broadband reflective polarizer in liquid crystal displays, it is desirable that the bandwidth of the reflective film should comprise a substantial portion of the visible wavelength range, whereas for an application as notch polarizer or as coloured reflective film e.g. for decorative or security applications, often films having a specific reflection colour are desired.
In particular broadband reflective polarizers, also known as circular polarizers, which are transmitting circularly polarized light of a broad wavelength band covering a large part of the visible spectrum, are suitable as polarizers for backlit liquid crystal displays.
If unpolarized light is incident on such a reflective polarizer, 50% of the light intensity are reflected as circularly polarized light with the same twist sense as that of the molecular helix, whereas the other 50% are transmitted. The reflected light is depolarized (or its sense of polarization is reversed) in the backlight of the display, and is redirected onto the polarizer. In this manner theoretically 100% of a given waveband of the unpolarized light incident on the reflective polarizer can be converted into circularly polarized light.
The circularly polarized light can be converted into linear polarized light by means of a quarter wave optical retarder and optionally also a compensation film.
A simple, but neither very effective nor economic way to provide a broadband reflective polarizer is to stack several reflective films with different reflection wavebands on top of each other. Recently reflective polarizers have been developed that comprise a liquid crystalline material with a helically twisted structure and a planar orientation, and are further characterized in that the pitch of the molecular helix is varying in a direction perpendicular to the layer, which leads to a large bandwidth of the reflected wavelength band.
Methods described so far for the preparation of broadband reflective polarizers from liquid crystalline precursors do have various drawbacks. The EP 0 606 940 (Broer et al.) discloses circular reflective polarizers with a bandwidth of up to 400 nm and their manufacture. This is realized by the exploitation of the diffusion of reactive mesogenes with different reactivity and chirality leading to a large variation of the cholesteric pitch, as disclosed in Broer et al. Nature, Vol. 378, pp. 467 (1995). However, this process is rather slow and in some cases even takes several minutes to complete. This is incompatible with most methods to fabricate polarizers on continuously moving substrates such as plastic films.
A process for the production of reflective films on plastic substrates is described in the WO 97/35219. Though this process is completed in the order of 15 to 30 seconds, and is thus faster than that used by Broer et al., it is nevertheless still relatively difficult with respect to the control of the resultant reflection wavelength and bandwidth of the reflective polarizer.
Furthermore, the methods described in the EP 0 606 940 and WO 97/35219 can only lead to spatially uniform characteristics of the reflective films, i.e. showing no variation of the pitch in lateral directions across the film. On the other hand, there are also applications where it is desired to have a reflective film with reflection characteristics that are spatially varying over the film, e.g. wherein different areas of the film show different reflection colours. These films are useful e.g. for information storage or as multicoloured images.
The GB 2,315,760-A discloses a polymerizable mesogenic composition that is thermochromic, i.e. it shows a change of the reflection colour upon temperature variation, and also discloses a method to prepare a multicoloured reflective film thereof, by coating the composition as a thin, oriented layer onto a substrate, selectively heating different regions of the layer to different temperatures (e.g. by means of a laser), so that they exhibit different reflection colours, and curing the different regions to fix the respective colour.
The method described in the GB 2,315,760-A, however, is still relatively complicated and time-consuming, as several heating and curing steps are required.
Consequently there was a need for a method to prepare reflective films with better and more easy control both of the reflection wavelength and the bandwidth of the film, as well as for a method to produce reflective films with spatially varying reflection wavelengths, wherein these films could be used as reflective polarizers, colour filters, or as coloured films for information storage or in decorative or security applications.
In connection with reflective films and optical polarization, compensation and retardation films as described in the present application, the following definition of terms are given.
The term xe2x80x98reflective filmxe2x80x99 as used in this application includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.
The term xe2x80x98helix axis perpendicular to the film planexe2x80x99 means that the helix axis is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes orientations where the helix axis is tilted at an angle of up to 2xc2x0 relative to the film normal.
The term xe2x80x98thermodynamically stable mesophasexe2x80x99 means the state that is obtained upon polymerization of a polymerizable mesogenic material, where the system during polymerization has sufficient time to relax to give the thermodynamically stable, highly ordered equilibrium mesophase of the polymerized material. The thermodynamically stable, equilibrium mesophase of the polymerized material can be achieved e.g. by polymerizing the mesogenic material in solution, or at low polymerization rates or to low molecular weights.
The term xe2x80x98homeotropic orientationxe2x80x99 means that the optical axis of the film is substantially perpendicular to the film plane, i.e. substantially parallel to the film normal. This definition also includes films wherein the optical axis is slightly tilted at an angle of up to 2xc2x0 relative to the film normal, and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel to the film normal.
The terms xe2x80x98tilted structurexe2x80x99 or xe2x80x98tilted orientationxe2x80x99 means that the optical axis of the film is tilted at an angle between 0 and 90 degrees relative to the film plane.
The term xe2x80x98splayed structurexe2x80x99 or xe2x80x98splayed orientationxe2x80x99 means a tilted orientation as defined above, wherein the tilt angle additionally varies monotonuously in the range from 0 to 90xc2x0, preferably from a minimum to a maximum value, in a direction perpendicular to the film plane.
The term xe2x80x98planar orientationxe2x80x99 means that the optical axis of the film is substantially parallel to the film plane. This definition also includes films wherein the optical axis is slightly tilted relative to the film plane, with an average tilt angle throughout the film of up to 1xc2x0, and which exhibit the same optical properties as a film wherein the optical axis is exactly parallel to the film plane.
In case the reflective polarizers and homeotropic, tilted, splayed, planar and twisted retardation and compensation films as defined above comprise uniaxially positive birefringent liquid crystal material with uniform orientation, the respective orientation of the optical axis corresponds to the orientation direction of the main molecular axes of the mesogens of the liquid crystal material.
The minimum and maximum wavelengths of the waveband reflected by an inventive reflective film, i.e. the edges of the band, in this application are not given as the values for half the values of the maximum of the bands. For practical reasons the minimum and maximum wavelengths are defined as those wavelengths on the given flank where the curve has the steepest slope in absolute values, compare FIGS. 5 to 8. The bandwidth is simply given as the difference between minimum and maximum wavelength. The central reflection wavelength also called short reflection wavelength or wavelength of reflection is given as the arithmetical average of the minimum and maximum wavelength.
One aim of the invention is to provide a method of manufacturing a reflective film that does not have the above mentioned drawbacks in an efficient and cost-effective manner which is in particular suitable for mass production. Other aims of the invention are immediately evident to a person skilled in the art from the following description.
The inventors have developed a technique that allows the preparation of a reflective film on plastic substrates and is also suitable for mass production. This method comprises the steps of coating a polymerizable liquid crystalline material with a chiral nematic or cholesteric phase on a substrate or between two substrates in form of a thin layer, aligning the material so that the cholesteric helix axis is perpendicular to the plane of the layer, and polymerizing the material to freeze in the helically twisted, planar liquid crystalline phase structure.
The inventors have found that the optical properties of a reflective film prepared by this process are sensitive to the method of production of the polarizer and the type of material used in this process. In particular, the inventors found that the helical pitch and reflection wavelength, i.e. the center of the reflection band, of the reflective film can be controlled by using a polymerizable material that has a less ordered mesophase at temperatures where the polymerized material has a more highly ordered, thermodynamically stable mesophase. Also, the new production method of the instant invention allows to determine and adjust the reflection wavelength of the film by appropriately selecting the composition of the polymerizable precursor mixture and/or by varying the irradiation power.
The above mentioned aims can be achieved and the drawbacks of prior art can be overcome with a reflective film that is obtainable by a process according to the present invention.
A broadband reflective polarizer prepared by a process according to the present invention is in particular advantageous in that, when used in a liquid crystal display, it exhibits a high luminance and a considerable brightness gain compared to a conventional linear polarizer (such as e.g. a dichroic polarizer) up to large viewing angles. Furthermore, it exhibits a high temperature stability of the mechanical and optical properties.
Further it is possible by the instant method to prepare polymer cholesteric liquid crystal films with a spatial (i.e. lateral) distribution of reflection wavelengths. Thus, the films can be patterned to reflect different colours in different areas simply by exposing these different areas to different irradiation powers. One simple method is to prepare a striped pattern by exposing a layer of polymerizable mesogenic material moving on a belt to the light of the illumination source, e.g. a UV lamp, which is dimmed to various degrees e.g. in a grating type of optics or by a graded mask.
One object of the present invention is a process of preparing a reflective film comprising a polymerized mesogenic material with helically twisted structure and varying helical pitch, by polymerizing a polymerizable mesogenic material in its helically twisted mesophase, characterized in that the pitch variation is achieved
by polymerizing at a temperature where the polymerizable material has a less ordered mesophase and the polymerized material has a more highly ordered thermodynamically stable mesophase.
and/or
by polmyerizing a polymerizable mesogenic material comprising
a) at least one polymerizable chiral or achiral mesogenic compound,
b) at least one chiral compound, which can be also be the compound of component a), d), e) or f),
c) at least one polymerization initiator,
d) optionally at least one crosslinking agent,
e) optionally at least one chain termination or chain transfer agent,
f) optionally a dye component comprising at least one dye, and varying amount and type of component c) and/or d) and/or e) and/or f).
Another object of the invention is a reflective film obtainable by a process as described in the foregoing and the following.
Another object of the invention is the use of an inventive reflective film as reflective broadband or notch polarizer or as a multicoloured film or image in liquid crystal displays, as colour filter, in effect pigments, for decorative or security applications.
Another object the invention is a liquid crystal display comprising a liquid crystal cell and a reflective polarizer as described in the foregoing and the following, and optionally further comprising at least one of the following components
I) an optical retardation film with a retardation which is approximately 0.25 times the central wavelength of the spectrum reflected by the reflective polarizer,
II) a linear polarizer,
III) a compensation film comprising a layer of an anisotropic polymer material with a homeotropic orientation,
IV) a compensation film comprising a layer of an anisotropic polymer material with a tilted or splayed orientation,
V) a compensation film comprising a layer of an anisotropic polymer material with a planar orientation,
VI) a compensation film comprising a layer of an anisotropic polymer material with a helically twisted structure, wherein the helix axis is perpendicular to the film plane.